Collecting solar energy, within the field of thermal collection, is increasingly taking more technological and economic importance from the point of view of hot water, heating or cooling production at the household level, as well as for electric energy production in solar thermal plants.
These systems require maximum solar energy absorption and the lowest possible energy losses. For this purpose, these are configured in the form of vacuum tubes or similar structures that reduce the losses by conduction and convection, and have highly power solar energy absorbing coatings, as well as low emission features in order to reduce energy losses by far-Infrared thermal radiation.
Consequently, both in the domestic side as in electric energy production, the selective absorbing coatings play an essential role and its proper functioning largely depends on the performance of this type of systems. This makes vitally important the fact of having a proper method for characterizing, in field, the optical features of said coatings. In the case of the electric energy production facilities, due to the large number of absorber tubes to be characterized, it is also convenient that the measurement could be quickly and easily performed.
Given the optical characteristics of this type of tubes (maximum energy absorption and minimum energy losses), the device must be capable of accurately measuring extreme values of the reflection and transmission coefficients (close to zero or unity), usually under unfavorable environmental conditions since, logically, the ambient light will be almost always high intense.
Since these reflection and transmission coefficients strongly depend on the wavelength of light used, it is indispensable to perform a spectral characterization thereof. A device performing a measurement of this kind is so-called spectrophotometer.
In a classical spectrophotometer a broad spectrum light source and a variable filter element are used, such as can be a movable diffraction grid followed by a narrow slit, which allows sequentially selecting different wavelengths. This option allows varying the wavelength in a practically continued manner, but instead it results in a more complex and delicate, and with low dynamic measurement range system, since the achieved input light power is very low.
The U.S. Pat. No. 4,687,329 describes a device that uses a broad spectrum source, in this case ultraviolet, and several filters in fixed positions for performing a spectral measurement in a certain number of discrete points.
Also there is a background of spectrophotometers in which a collection of sources with different wavelengths is used as a light source. In the patent US2008/0144004 several light emitting diodes (LED) are simultaneously used for performing a transmission measurement in order to detect different analytes in blood. However, a true spectral measurement is not performed, but several simultaneous measurements in a few different wavelengths. In addition, there is no protection against ambient light or the possibility of performing reflection or reference measurements.
Something similar occurs in the invention of the U.S. Pat. No. 4,286,327, wherein a sequential measurement at different wavelengths (in the infrared) is performed, but in this case the used LEDs are identical and the spectral selection is done through fixed filters with different central wavelengths. Nor is there any mechanism for recovering the signal against ambient light, or the possibility of performing reflection or reference measurements.
None of the mentioned or other similar devices meet the requirements necessary of the in-field measure of absorbing tubes for solar collectors, either by range, sensitivity and/or mechanical configuration.